The function of a linear power amplifier is to amplify a signal with as little waveshape alteration as is practical. The ideal amplifier is therefore characterized as having a transfer function (input signal compared to output signal) which is completely linear with no transfer function discontinuities. Unfortunately, physical processes are seldom ideal and signal amplifiers are no exception. Amplifiers are specifically designed to operate as linearly as possible within their "linear region," but amplifier nonlinearities are a reality in any amplifier design. Additionally, amplifiers which are "overdriven" deliver a clipped output signal. An amplifier is overdriven, and therefore the output signal is clipped, when the input signal possesses peak amplitudes which cause the amplifier to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude). Generally, an amplifier is characterized as having a "clipping threshold." Input signals having amplitudes beyond the clipping threshold are clipped at the amplifier output.
One method of ameliorating the effects of nonlinear amplifier performance within the amplifier "linear region" is to intentionally distort the preamplified RF signal to anticipate and complement the recognized nonlinear trait of the amplifier. The device which performs this function is known as a signal predistorter (hereinafter referred to as a "predistorter"). Since the departure from linearity of an amplifier operating in its "linear region" is known (as characterized by a deviation of its transfer function in the "linear region" from that of an ideal amplifier's transfer function), a predistorter intentionally distorts a preamplified signal by compensating for the known nonlinearity in a complementary fashion. Thus, when the intentionally distorted preamplified signal is amplified, the nonlinearity of the amplifier causes the amplified version of the intentionally distorted signal to more closely resemble the waveshape of the original signal (the signal prior to amplification and predistortion). Predistortion is thus an effective method for compensating for amplifier nonlinearities within the "linear region" of an amplifier, and as such is frequently referred to as an amplifier linearization circuit.
However, conventional amplifier linearization (predistortion) techniques do not compensate for the resultant adverse effects when the amplified signal nonlinearity is the result of clipping. In a wireless RF transmitter, the presence of signal clipping at the power amplification stage presents an especially onerous problem. Specifically, clipping of a RF signal typically results in significant quantities of spectral regrowth (emission of RF signal energy outside the intended frequency band). In a wireless RF environment, where a high priority is placed upon effective and efficient utilization of limited bandwidth, the production of spectral regrowth causes RF interference emissions outside of the intended (or allocated) frequency band and therefore noise (interference) within unintended (nonallocated) frequency bands. The severity of these out-of-band emissions is proportional to the shortness and abruptness with which the clipping of the RF signal waveform occurs.
In the prior art, compensation for clipping within a power amplifier involved generating a compensating window to apply to a RF signal and multiplying a predetermined windowing function with the RF signal to be amplified. This method, disclosed in U.S. Pat. Nos. 5,287,387 and 5,638,403, is operable to reduces, but does not eliminate, spectral regrowth. Therefore, there exists a need for an improved method and device with which to further minimize the deleterious effects of clipping and other nonlinearities which occur during signal amplification.